322 ESSENTIALS OF BOTANY 
or by continued selection first of a set of choice parent 
plants, then of their best offspring, and so on for several 
generations. 
Hybridizing sometimes (but not nearly always) aids the 
plant breeder by giving him a large number of marked 
variations from which 
to select. 
High cultivation 
together with plant 
breeding have brought 
about many astonish- 
ing results. Plums 
three inches long have 
recently been produced. 
A hybrid beach-plum 
bears so abundantly 
that the twigs are liter- 
ally hidden by the fruit. 
The largest cultivated 
apples are many hun- 
The Bismarck apple, with a, the wild Asiatic PP é y 
crab apple (Pyrus baccata), and b, the Euro- dred times the bulk of 
pean wild apple (P. malus). (All half nat- their remote wild an- 
ural size.) 
Fig. 225. Effect of Cultivation upon the 
Size of Apples. 
cestors. A new variety 
of blackberry plant covers one hundred and fifty square 
feet of soil and bears a bushel or more of fruit. Most culti- 
vated roots and tubers have been greatly changed from 
their wild condition, losing in the proportion of woody 
fiber which they contain and gaining immensely in size 
(Fig. 221). 
One of the most important problems for the plant 
breeder is how to secure varieties immune to diseases. 
Two of the most notable achievements of our Department 
